Every Cloudflare installation can secure both your domain and all of its subdomains without additional cost. All you need to do is enable Cloudflare protection for subdomains for this to happen.
Option 1 - Your domain is purchased at Hostinger
Open your Cloudflare management page. There, scroll down to the Subdomains section. Just add a checkmark near the subdomains which you would like to protect with Cloudflare, and save changes:
If your domain is not pointing to Cloudflare by nameservers yet (it’s done automatically for all new Cloudflare activations), you will need to migrate to Cloudflare DNS.
Migrating to Cloudflare DNS
On the subdomains section of your Cloudflare management page, click on Migrate:
After this, your domain will be moved to Cloudflare DNS, and protecting subdomains with Cloudflare will be unlocked.
Option 2 - Your domain is purchased elsewhere
If your domain is purchased elsewhere, while activating Cloudflare, make sure to point domain to Cloudflare nameservers. After the propagation is completed, just add a checkmark near subdomains which you would like to set up a protection for.
Make sure that your subdomain has an SSL certificate before doing this change!
Your SSL/Encrypt communication (mode) at Cloudflare Dashboard should be set to Full